The unmanned Falcon 9 rocket launched by SpaceX on a cargo resupply service mission to the International Space Station lifts off from the Cape Canaveral Air Force Station in Florida Saturday morning.

Scott Audette/REUTERS

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Some 2.5 tons of freight are speeding toward the International Space Station following Saturday morning's successful launch of SpaceX's Falcon 9 rocket topped with the company's Dragon cargo capsule.

The rocket launched at 4:47 a.m. Eastern Standard Time from the Cape Canaveral Air Force Station in Florida, following an aborted launch attempt Jan. 6. That launch was scrubbed less than two minutes before lift-off after launch controllers reported that a key component in the steering mechanism for the rocket's second stage wasn't working properly.

This morning's launch went flawlessly, delivering the capsule to orbit some 17 minutes after launch. It's the company’s fifth formal cargo flight to the station under a $1.6 billion agreement with NASA to resupply the space station.

Although the mission's primary goal is to deliver the goods to the station, the launch also represented Space Exploration Technology Corporation's first try at returning a first-stage booster safely back to Earth. In this case, Earth was represented by a football-field-size, ocean-going platform dubbed the autonomous spaceport drone ship.

Three times before, Falcon 9 boosters had soft-landed into the ocean during initial tests of the booster-return system. This time, with landing legs added, the stage was to have set down on the platform.

Instead, tweeted SpaceX CEO and chief technology officer Elon Musk, "Rocket made it to drone spaceport ship, but landed hard. Close, but no cigar this time. Bodes well for the future tho."

Even so, landing with a resounding thud could be considered a partial success. The SpaceX team delivered the first stage to the platform. In past tests, from about 150 miles up, the first stages splashed down within about six miles of the projected landing spot. Engineers added special fins to the first stage of the Falcon 9 used Saturday to help steer it to a more-accurate landing. The goal was to land with an accuracy of about 30 feet. A hard landing on the platform suggests that the fin system worked.

The company had another, crewed vessel nearby, but it was too dark and foggy to get decent video of the landing attempt. Still, engineers have a wealth of telemetry the first stage sent throughout its descent they can analyze for clues as to what changes need to be made to improve chances for success on future launches.

SpaceX is trying to perfect the system so that it can use a first stage for multiple launches. The goal is to drive down launch costs in hopes of expanding access to space for a wider variety of potential users.

The Falcon 9, as well as the more-powerful Falcon Heavy slated for its initial demonstration flight later this year, are unlikely to sport reusable seconds stages, Mr. Musk acknowledged in a question-and-answer session on reddit.com earlier this week.

When asked about reusable second stages, he replied that the second stage could be modified to be reusable without sacrificing much in the way of payload capacity, especially for the Falcon Heavy.

"But I think our engineering resources are better spent moving on to the Mars system," he added, referring to a Mars transportation system he has in mind.

For now, however, the immediate focus is on Dragon's trip to the space station.

Over the next two days, the craft will incrementally adjust its orbit until it catches up with the space station. The capsule is slated to arrive at the station on Jan. 12, where it will remain until Feb. 10, when it's scheduled to return to Earth in a water landing off the coast of Baja California.

Dragon is carrying 5,108 pounds of food, water, clothing, and science experiments to the space station. Among the experiments: laser-based detector designed to trace the movement of clouds and tiny particles known as aerosols as they move around in the atmosphere. The aerosols come in a variety of forms – dust, pollutants, and smoke from wildfires, among others.

These particles can have a significant effect on cloud formation and on the amount of rain or snow clouds can deliver. Aerosols also have been tied to a range of public-health problems.

Dragon is also carrying a set of 17 experiments designed by students from around the US.

The experiment package, formally known as Yankee Clipper, initially was to be delivered to the station by Orbital Science Corporation, the second company under contract to NASA to deliver cargo to the space station, on its third resupply mission Oct. 28. But the rocket carrying the cargo exploded less than 20 seconds after launch. The students were given an opportunity to send their experiments up on Dragon.

Dragon is the only space-station resupply craft that was designed to return cargo to Earth as well as deliver it to the orbiting outpost – a return capability that the US lost briefly when it ended the space-shuttle program in 2011. On the trip home, the capsule will be hauling back some 3,600 pounds of cargo.